The invention relates generally to X-ray tubes, and more particularly to structures and methods of assembly for the shaft of the anode utilized in an X-ray tube.
X-ray systems may include an x-ray tube, a detector, and a support structure for the x-ray tube and the detector. In operation, an imaging table, on which an object is positioned, may be located between the x-ray tube and the detector. The x-ray tube typically emits radiation, such as x-rays, toward the object. The radiation passes through the object on the imaging table and impinges on the detector. As radiation passes through the object, internal structures of the object cause spatial variances in the radiation received at the detector. The detector then emits data received, and the system translates the radiation variances into an image, which may be used to evaluate the internal structure of the object. The object may include, but is not limited to, a patient in a medical imaging procedure and an inanimate object as in, for instance, a package in an x-ray scanner or computed tomography (CT) package scanner.
X-ray tubes include a cathode and an anode located within a high-vacuum environment. The anode structure is typically supported by one or more bearing members, such as ball bearings, and is rotated for the purpose of distributing the heat generated at a focal spot. Typically, an induction motor is employed to rotate the anode, the induction motor having a cylindrical rotor built into a cantilevered axle that supports a disc-shaped anode target and an iron stator structure with copper windings that surrounds an elongated neck of the x-ray tube. The rotor of the rotating anode assembly is driven by the stator. An x-ray tube cathode provides a focused electron beam that is accelerated across an anode-to-cathode vacuum gap and produces x-rays upon impact with the anode. Because of the high temperatures generated when the electron beam strikes the target, it is necessary to rotate the anode assembly at high rotational speed. This places stringent demands on the bearings and the material forming the anode structure, i.e., the anode target and the shaft supporting the target.
In other constructions, a liquid metal bearing may be employed in lieu of ball bearings. Advantages of liquid metal bearings include a high load capability and a high heat transfer capability due to an increased amount of contact area as compared to a ball bearing. Advantages also include low acoustic noise operation as is commonly understood in the art. Gallium, indium, or tin alloys are typically used as the liquid metal, as they tend to be liquid at room temperature and have adequately low vapor pressure, at operating temperatures, to meet the rigorous high vacuum requirements of an x-ray tube. However, liquid metals tend to be highly reactive and corrosive. Thus, a base metal that is resistant to such corrosion is desirable for the components that come into contact with the liquid metal bearing, such as the shaft of the anode assembly.
As a result, in either construction, the structure of the shaft to which the anode is connected must be capable of withstanding the high temperatures and mechanical stresses created within the x-ray tube, as well as be able to withstand the corrosive effects of the liquid metal bearing. As such, a refractory metal such as molybdenum or tungsten is typically used as the base material for the construction of the shaft as well as for the bearing components. Not only are such materials resistant to corrosion and high temperatures, but they tend to be vacuum-compatible and thus lend themselves to an x-ray tube application.
However, rather than have a shaft formed of a single material, it is desirable in many situations to form the shaft of different materials, each material having properties suited to the particular application or position of the shaft portion within the x-ray tube. For example, the material used for the thrust and journal surfaces of the bearing shaft must exhibit minimal reaction with the any liquid metal bearing fluid at the temperatures experienced during bearing processing and operation. However, the optimum materials used for the bearing surfaces are different from those used for the welding to the x-ray tube assembly. As a result, the dissimilar materials need to be hermetically joined in order to form the shaft.
One technique for minimizing base material expense and improving functionality is to include the preferred base metal (i.e., tungsten or molybdenum) only in regions that require the characteristics of the particular base metal. An extension made of a less expensive material may then be brazed thereto, the extension serving as a mechanical connection as support for an anode. In other words, as an example, a stationary center shall may support a rotatable support structure having an anode attached thereto. The center shaft may be made entirely of the preferred base metal, or the cost thereof may be reduced by attaching a less expensive steel thereto via brazing, thus reducing the total amount of the preferred base metal. Such a design may result in cost savings because of the less expensive steel portion being used in lieu of the preferred base metal. However, cost savings achieved while using this technique are typically offset to an extent by the additional attachment processing, such as by attaching the extension thereto having a hermetic seal.
However, when using brazing as the method for joining the dissimilar materials together very high temperatures are required, which can negatively affect the mechanical properties of the materials being joined, thereby reducing the tube life.
In an alternative method, the shaft can be formed by friction welding where the primary weld surfaces of the dissimilar materials are brought together under intense pressure and relative motion creating conditions (heat, mechanics, etc.) allowing these surfaces of the differing materials to metallurgically bond in solid state. An example of this is shown in U.S. Pat. No. 5,592,525. Traditional friction welding requires specific localized conditions that drive the bonding in the materials making up the primary welding surfaces. As the material properties of the primary surface materials diverge, the complexity of process design becomes complex and often infeasable and ultimately uneconomical. In short, friction welding is limited to particular material combinations and requires significant mechanical energy input to achieve quality joints and can degrade the properties of the base materials due to phenomenon such as grain growth.
As a result, it is desirable to develop a structure and method for the formation of a bearing shaft for an x-ray tube anode that can be formed with dissimilar materials but without degrading the desirable properties of the materials being joined.